I'm new in golang development and have some question regarding something related to this question.
As a learning exercise, I'm trying to create a simple library to handle json based configuration file. As a configuration file to be used for more then one app, it should be able to handle different parameters. Then I have created a type struct Configuration that has the filename and a data interface. Each app will have a struct based on its configuration needs.
In the code bellow, I put all together (lib and "main code") and the "TestData struct" is the "app parameters". If it doesn't exists, it will set a default values and create the file, and it is working. But when I try to read the file. I try to decode the json and put it back into the data interface. But it is giving me an error and I couldn't figure out how to solve this. Can someone help on this?
[updated] I didn't put the targeted code before, because I though that it would be easier to read in in all as a single program. Bellow is the 'targeted code' for better view of the issue. As I will not be able to use the TestData struct inside the library, since it will change from program to program, the only way to handle this was using interface. Is there a better way?
library config
package config
import (
"encoding/json"
"fmt"
"os"
)
// Base configuration struct
type Configuration struct {
Filename string
Data interface{}
}
func (c *Configuration) Create(cData *Configuration) bool {
cFile, err := os.Open(cData.Filename)
defer cFile.Close()
if err == nil {
fmt.Println("Error(1) trying to create a configuration file. File '", cData.Filename, "' may already exist...")
return false
}
cFile, err = os.Create(cData.Filename)
if err != nil {
fmt.Println("Error(2) trying to create a configuration file. File '", cData.Filename, "' may already exist...")
return false
}
buffer, _ := json.MarshalIndent(cData.Data, "", "")
cFile.Write(buffer)
return true
}
func (c *Configuration) Read(cData *Configuration) bool {
cFile, err := os.Open(cData.Filename)
defer cFile.Close()
if err != nil {
fmt.Println("Error(1) trying to read a configuration file. File '", cData.Filename, "' may not already exist...")
return false
}
jConfig := json.NewDecoder(cFile)
jerr := jConfig.Decode(&cData.Data)
if jerr != nil {
panic(jerr)
}
return true
}
program using library config
package main
import (
"fmt"
"./config"
)
// struct basic para configuração
type TestData struct {
URL string
Port string
}
func main() {
var Config config.Configuration
Config.Filename = "config.json"
if !Config.Read(&Config) {
Config.Data = TestData{"http", "8080"}
Config.Create(&Config)
}
fmt.Println(Config.Data)
TestData1 := &TestData{}
TestData1 = Config.Data.(*TestData) // error, why?
fmt.Println(TestData1.URL)
}
NEW UPDATE: I have made some changes after JimB comment about I'm not clear about some concepts and I tried to review it. Sure many things aren't clear for me yet unfortunately. The "big" understanding I believe I got, but what mess my mind up is the "ins" and "outs" of values and formats and pointers, mainly when it goes to other libraries. I'm not able yet to follow the "full path" of it.
Yet, I believe I had some improvement on my code.
I think that I have corrected some points, but still have some big questions:
- I stopped sending "Configuration" as a parameter as all "data" were already there as they are "thenselfs" in the instance. Right?
- Why do I have use reference in the line 58 (Config.Data = &TestData{})
- Why to I have to use pointer in the line 64 (tmp := Config.Data.(*TestData)
- Why I CANNOT use reference in line 69 (Config.Data = tmp)
Thanks